Heat shrinkable labels, which are prepared by printing images on a base film made of heat shrinkable resin, are attached to various containers such as plastic bottles and metal cans these days.
For such heat shrinkable labels, polystyrene resin films are often used because of their excellent low-temperature shrinkability. However, polystyrene resin films are insufficient in heat resistance and solvent resistance. To solve this problem, polyester resin films with excellent heat resistance and solvent resistance have also been used. However, polyester resin films have poor low-temperature shrinkability and rapidly shrink, so that they easily wrinkle when attached to containers. Moreover, polyester resin films used as heat shrinkable labels are not readily torn along perforations provided for easy removal of the film from containers for recycling after use.
To solve such problems, for example, there have been studied multilayer films including polyester resin-containing front and back layers and a polystyrene resin-containing interlayer. Here, in multilayer films, it is an important issue to prevent delamination.
To prevent the delamination, adhesive layers are provided between the front and back layers and the interlayer. Patent Literature 1 discloses a heat shrinkable multilayer film in which an adhesive resin for adhesive layers is a resin mixture containing a polyester resin and a polystyrene resin. In such a heat shrinkable multilayer film, however, the adhesive strength between the front and back layers and the interlayer is low, that is, the adhesive strength between layers is not enough.
Patent Literature 2 discloses a heat shrinkable laminated film in which an adhesive resin contained in adhesive layers is a soft polystyrene resin having a styrene content of 10 to 50%, a modified styrene resin having a large elastomer content, a resin that has high affinity to and is compatible with polyesters, or a mixture of these.
These heat shrinkable laminated films however have a problem of insufficient adhesive strength between layers because the adhesive strength between the front and back layers and the interlayer is lowered after printing for use as a label.
Patent Literature 3 discloses a heat shrinkable multilayer film including a polyester elastomer as a component of adhesive layers.
However, such a heat shrinkable multilayer film sometimes have residual white streaks along creases, which are caused by folding with a strong pressure during center sealing in production of a heat shrinkable label. The creases remain even after the heat shrinkable label is placed on a container and heat-shrunk, resulting in impaired appearance of the film.
Even in the case of a heat shrinkable label including a multilayer film that has front and back layers each containing a polyester resin, an interlayer containing a polystyrene resin, and adhesive layers provided therebetween, when the heat shrinkable label is attached to a plastic bottle after storage in an low-temperature environment (e.g., storage in winter) or when the attachment is performed at low temperature, delamination may occur in the heat shrinkable label upon heat shrinkage, leading to defective appearance or detachment of the heat shrinkable label from the plastic bottle.